Linear drive shafts are used in many mechanical applications where linear (to and fro) motion is necessary to perform a task. Such linear drive shafts are driven by a variety of driving means; for example, hydraulic pressure against a spring-loaded plunger is one effective linear actuator. Other linear drive means include solenoids, shuttle rods, and cam/follower arrangements. In simple systems, linear-drive shafts can also be manually operated.
A major disadvantage of linear drive shafts is that such shafts are difficult to seal effectively. Where a linear drive shaft is expected to operate in a contaminant-free or evacuated environment, it is often not practical to include the driving means within the work chamber because of either the bulk of the driving means or because it is, itself, a source of contaminants. In such cases it is then necessary to seal the drive shaft at its point of entry into the work chamber.
Sealing a sliding shaft is a difficult task. One conventional method of sealing a shaft is to surround the shaft with a close-fitting sleeve and then fill the annular space between the shaft and the sleeve with a high-density lubricant. This solution is often impractical in evacuated work chambers and is less than satisfactory in other situations as well because the lubricant tends to be forced out of the annular space between the sleeve and the shaft as a result of the to-and-fro motion.
An improvement on the sleeve and lubricant sealing apparatus calls for the use of a magnetic fluid in conjunction with a magnetized shaft and a magnetically permeable sleeve (or vice-versa, a magnetized sleeve and magnetically permeable shaft). While this arrangement improves the sealing capabilities of the sleeve and shaft apparatus, particularly in evacuated systems, this improvement also suffers from the problem of eventual migration of the lubricant out of the annular space between the sleeve and shaft.
Therefore, it is desirable to provide for a simple, effective, linear drive-shaft sealing apparatus, particularly one which is adapted for use with high-vacuum, contaminant-free environments.
One particular application where linear drive shafts play an important role is in the fabrication of high-quality, semiconducting, piezoelectric and optical crystals. Such crystals are typically formed from a supersaturated melt solution by slowly removing a crystal seed from the melt. In fabricating such crystals, it is essential that the seed be removed in a controlled manner to ensure continuous growth with an orderly crystal lattice structure. It is also essential that the crystal-growing operation be conducted in a contaminant-free, high-vacuum environment so that the crystals formed are of the utmost purity.
Therefore, it would also be desirable to provide for an improved linear drive-shaft sealing apparatus for use in conjunction with crystal-growing operations.